1. Field of the Invention
The present invention relates to a method for driving a liquid crystal display (hereinafter referred simply to as an LCD), its driving circuits and an image display device and more particularly to the method for driving the LCD which is used as a display device for a personal computer or a like and in which liquid crystal cells are arranged in a matrix form, to its driving circuits and the image display device equipped with such the driving circuits for the LCD.
The present application claims priority of Japanese Patent Application No. 2000-244963 filed on Aug. 11, 2000, which is hereby incorporated by reference.
2. Description of the Related Art
FIG. 12 is a schematic block diagram showing an example of configurations of a driving circuit of a conventional color LCD 41 disclosed in Japanese Laid-open Patent Application No. Hei 03-083014. Hereinafter, the disclosed technology is called a first conventional example.
The color LCD 41 of the first conventional example is an active-matrix color LCD using, for example, a TFT (Thin Film Transistor) as a switching element. In the color LCD 41, each of pixel portions is mounted at an intersection of each of a plurality of scanning electrodes 42 (gate lines) placed at specified intervals in a row direction and each of signal electrodes 43 (source lines) placed at specified intervals in a column direction. Moreover, in each pixel portion, a liquid crystal cell 44 being equivalently a capacitive load, a TFT 45 whose drain is connected to one terminal of a corresponding liquid crystal cell 44 and a capacitor 46 being connected in parallel to a corresponding liquid crystal cell 44 and storing a signal electric charge for one vertical sync period are provided. In a state in which a common electrode VCOM is applied to all liquid crystal cells 44 and capacitors 46 being all connected in parallel, when data signal SD produced based on a video red signal SR, video green signal SG, and video blue signal SB is applied to each of the signal electrode 43 and when a scanning signal produced based on a horizontal sync signal SH and a vertical sync signal SV is applied to each of the scanning electrode 42, a color character, color image, or a like is displayed. On the color LCD 41, for example, as shown in FIGS. 13A and 13B, color filters for a red color (R), green color (G), and blue color (B) making up three primary colors, each corresponding to each of the liquid crystal cells 44, are arranged. In the example shown in FIGS. 13A and 13B, since each of the color filters for the R, G, and B colors is so arranged that each of them is deviated by a half of a pitch from a place of a subsequent scanning line and a dot pixel portion constructed of the three color filter portions for the R, G, and B colors making up one pixel portion is of a triangular shape, such the arrangement is called a delta shape or a triangular shape arrangement. That is, in the color LCD 41, one pixel portion is made up of three color filter portions containing the R color filter, G color filter, and B color filter each corresponding to each of the liquid crystal cells 44.
Moreover, the driving circuit for the color LCD 41 of the first conventional example, as shown in FIG. 12, chiefly includes a controller 51, a signal electrode driving circuit 52, and a scanning electrode driving circuit 53. The controller 51 feeds the video red signal SR, video green signal SG, and video blue signal SB, all of which are supplied from outside, to the signal electrode driving circuit 52 and, at the same time, produces a horizontal scanning pulse PH and a polarity reversing pulse POL used to drive the color LCD 41 with alternating current, based on the horizontal sync signal SH and vertical sync signal SV, all of which are supplied from outside and feeds them to the signal electrode driving circuit 52 and also produces a vertical scanning pulse PV, based on the horizontal sync signal SH and vertical sync signal SV, all of which are supplied from outside, and then feeds it to the scanning electrode driving circuit 53. The signal electrode driving circuit 52 produces, with a timing when the vertical scanning pulse PH is fed from the controller 51, the data signals SD using the video red signal SR, video green signal SG, and video blue signal SB and, after having reversed or having not reversed the polarity of the data signals SD based on the polarity reversing pulse POL, feeds each of them to each of corresponding signal electrodes 43 in the color LCD 41. The scanning electrode driving circuit 53 produces, with a timing when the vertical scanning pulse PV is fed from the controller 51, scanning signals and feeds each of the scanning signals to each of corresponding scanning electrode 42 in the color LCD 41.
The color LCD 41 in which each of the color filters for the R, G, and B colors is arranged in a delta form as shown in FIGS. 13A and 13B is driven in a manner that the polarity of each of the data signal SD to be fed to each of the signal electrode 43 is reversed for one scanning electrode 42 of the color LCD 41, that is, in every scanning period and for every pixel portion forming the delta shape existing adjacent to a direction of scanning. Since a change in luminance in a frame occurs in a delta form, this driving method is called a delta reversing driving method. FIGS. 13A and 13B show that, in the color LCD being in a state of different color connection in which the TFT 45 to drive the liquid crystal cell 44 making up the dot pixel portions composed of different colors is connected to one signal electrode 43, the data signal to be applied to the TFT 45 to drive the liquid crystal cell 44 making up the dot pixel portions existing at a portion surrounded by sloped lines is of positive polarity and the data signal to be applied to the TFT 45 to drive the liquid crystal cell 44 making up the dot pixel portions existing at a portion other than that surrounded by the sloped lines is of negative polarity and that switching is done between one state shown in FIG. 13A and the other state shown in FIG. 13B in every frame period. The reason for using the frame period is that, since the color LCD 41 employs a non-interlace method, the period employed is made associated with a field period employed in the NTSC (National Television System Committee) using an ordinary interlace-type display.
In the color LCD 41 of the example, not only since a pixel pitch between vertical stripes occurring in a frame on a display screen of the LCD 41 is narrow, but also since the vertical stripes are nested together with each other, a state in which differences in colors are not perceptible with human eyes is produced and a flicker in a white color display can be reduced.
Moreover, another method for driving the conventional LCD is disclosed in Japanese Laid-open Patent Application No. Hei 03-078390 in which one pixel portion is made up of four dot pixel portions having color filters for the G, G, R, and B colors arranged in a quadrangular form and an LCD is made up of a plurality of the pixel portions arranged in a matrix form. In this LCD, when a polarity of a data signal SD to be fed to a signal electrode connected to each of the dot pixel portions is reversed during a frame period, the data signal SD is controlled so that, in a same frame, the data signal to be fed to the R and G dot pixel portions and the data signal to be fed to the B and G dot pixel portions are opposite in polarity and also the data signal to be fed to the G and G dot pixel portions and the data signal to be fed to the R and B dot pixel portions are opposite in polarity. Hereinafter, the disclosed technology is called a second conventional example. FIGS. 14A and 14B show that the data signal to be applied to the TFT used to drive the liquid crystal cell making up the dot pixel portions existing at a portion surrounded by sloped lines is of positive polarity and the data signal to be applied to the TFT used to drive the liquid crystal cell making up the dot pixel portions existing at a portion other than that surrounded by the sloped lines is of negative polarity and that switching is done between one state shown in FIG. 14A and the other state shown in FIG. 14B in every frame period.
In the LCD of the second conventional example, a state of the occurrence of the flicker being stripes having different hues changes alternately and a spatial pitch among the flickers is made small and a line flicker being visually identified as if the scanning line were to sway right and left by changes, with time, of vertical stripes of light and shade occurring in a frame and a face flicker being visually identified as if there were to be light and shade portions on an entire screen during a frame period can be reduced.
However, the above technology of the first conventional example has a problem. In the LCD of the first conventional example, when a red monochromatic color is displayed, states shown in FIG. 15A and FIG. 15B occur. Generally, even when data signals being at a same potential but being different in polarity are fed to a signal electrode in order to drive a color LCD with alternating current, since, due to a characteristic of the TFT constructed of amorphous silicon, an on-current flowing when the data signal of negative polarity is applied is smaller than an on-current flowing when the data signal of positive polarity is allied, an unbalance is produced between when the data signal having a current of negative polarity flowing through a drain of the TFT is applied and when the data signal of positive polarity is applied. Because of this, when luminance at the “a” portion shown in FIG. 15A is compared with that at the “b” portion shown in FIG. 15A, since the red monochromatic color is displayed originally, though a same data signal being different only in its polarity is applied to a corresponding signal electrode to display the same red color with a same luminance, the luminance at the “a” portion is slightly darker than that at the “b” portion. Moreover, as described above, since the polarity of the data signal to be applied to the TFT corresponding to the “a” and “b” portions is reversed in every frame period (between FIG. 15A and FIG. 15B), the difference in the luminance between at the “a” portion and at the “b” portion is visually identified by a user as the line flicker having vertical stripes of light and shade with one half of frame frequencies. As a result, there is a defect that the flicker cannot be reduced when an image in any monochromatic color is displayed or when an arbitrary image in colors other than the white color is displayed.
Moreover, the LCD of the first conventional example has another shortcoming. That is, when an adjuster, while visually identifying the line flicker that has already occurred, adjusts the common potential VCOM so that the line flicker can be minimized, it is possible to make the adjustment that can minimize the line flicker only in a local region of the entire display screen, however, it is impossible to make the adjustment that can minimize the flickers occurring on the entire display screen. Thus, if the adjustment for optimizing the common voltage VCOM cannot be made, since a balance between the potential of the data signal of positive polarity and that of the data signal of negative polarity to be used to drive the color LCD with alternating current is lost due to a deviation of the common potential VCOM. This causes a phenomenon called image persistence in which a trace of the character or the like remains left on the screen even after power is turned OFF, caused by a long time display of same characters or the like on the screen.
On the other hand, the LCD of the second conventional example also has a problem. That is, since one pixel portion is made up of the four dot pixel portions, the number of liquid crystal cells each corresponding to each of the dot pixel portions of the TFTs used to drive the liquid crystal cells and of the capacitors used to accumulate signal charges is larger by about 1.3 times than that in the case where one pixel is made up of the three dot pixel portions and the color filters corresponding to the dot pixel portions are arranged in the stripe form, as shown in FIG. 16. This causes a yield in production of the LCD to be decreased, manufacturing costs to be increased, and the LCD to become expensive. Moreover, since the elements such as the liquid crystal cells or the like increase, if the same image as displayed in the LCD in which the dot pixel portions are arranged in the stripe form shown in FIG. 16 has to be displayed in the LCD shown in FIGS. 14A and 14B within a same time, signals have to be processed at a high speed, mathematically, being higher by about 1.3 times. Therefore, such the LCD cannot be applied to recent application areas in which the display is made more high-definition and the screen is made larger, which require high signal processing.